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- Strain = relative change in fracture gap over original gap
- If strain < 2% = absolute stability = primary bone healing
- If strain 2-10% = secondary bone healing
- If strain 10-17% = fibrous nonunion
- If strain > 17% = nonunion with granulation tissue
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- NO FRACTURE CALLUS
- Primary bone healing occurs via 2 mechanisms:
- Contact Healing [gap <0.01mm] → cutting cones form directly without intermediate bone formation
- Gap Healing [gap <1mm] → direct lamellar bone formation and subsequent remodeling with cutting cones
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- 3 zones - cutting zone, reversal zone, closing zone
- Cutting zone - osteoclasts cut into the bone
- Reversal zone - transition area with increasing osteoblasts
- Contains precursor cells and capillaries for nutrition
- Closing zone
- Mainly osteoblasts lining the cone and laying down bone
- Osteoblasts become entombed in new bone to form osteocytes
- Process: Osteoclastic tunneling, followed by capillary formation and osteoblast bone formation
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- Inflammatory phase – hematoma formation with cytokine release, cell migration, and angiogenesis
- Soft callus formation occurs via:
- Intramembranous ossification under periosteum at bone edges. Osteoid (type 1 collagen) is laid down directly by periosteal osteoblasts in the inner cambium layer. Forms hard callus but doesn't bridge the fracture site. (bone forms without cartilage via osteoprogenitor cells)
- Endochondral ossification (cartilage formation first by chondroblasts) - produces type 2 cartilaginous collagen.
- Hard callus formation - chondroclasts break down callus while osteoblasts lay down osteoid
- Remodeling phase - woven bone converts to lamellar bone via cutting cones
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- Giannoudis proposed the "diamond concept," which divides healing factors into osteoinductive, osteogenic, osteoconductive matrix, and mechanical stability
- Osteoinductive - growth factors like BMP when indicated
- Osteogenic - presence of bone grafts
- Osteoconductive - structural graft support
- Mechanical stability - implant choice and fixation technique
- Host factors - comorbidities like DM, steroid use
- Vascularity - peripheral vascular disease
- These 6 factors together create a "bio-reactor"
giannoudis2007.pdf111.3KB
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- A recent theory by Elliot et al. proposing that tissue forming around a fracture should be viewed as a specific functional entity - the "bone healing unit" (BHU). This unit responds to forces and follows various principles including Perren's strain theory, Wolff's law, and the "mechanostat"
elliott2016.pdf1972.1KB
NON-UNION
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- According to the FDA's 1988 definition, non-union occurs when a fracture has either not healed after 9 months, or shown no progress in the last 3 months
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- Infected
- Aseptic
- Hypertrophic = stability problem
- Atrophic = biological problem
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- Following Giannoudis's diamond concept, we need to consider these factors:
- Osteogenesis - optimize patient factors (DM, PVD) to ensure good vascularity; use autologous bone grafts to support osteogenesis, osteoinduction, and osteoconduction
- Osteoinductive - use of BMP 2 and 7
- Osteoconductive - use of autografts
- Stability - augment fixation with plate or exchange nail
- Example - femur shaft fracture non-union
- Rule out infection - bloods, Metsemaker's criteria
- Hypertrophic non-union indicates a stability issue
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- In this case, despite having a stiff fixation with short working length and high screw density, the fracture progressed to hypertrophic non-union. This X-ray was taken 1 year after surgery.
- The hypertrophy likely developed after screw breakage led to increased instability
BONE GRAFTING
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- OG, OI, OC
AUTOGRAFTS
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- Autografts are classified as cortical, cancellous, corticocancellous, or vascularized bone grafts
- Cancellous bone - rich in mesenchymal stem cells with high osteogenic and osteoconductive potential; trabecular structure allows rapid revascularization
- Cortical - provides better structural support but revascularizes more slowly, with lower osteogenic and osteoinductive properties
- Corticocancellous - combines advantages of both types
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- Depends on type of bone
- Cancellous bone = By creeping substitution (3 stages) [No callus as no endochondral ossification]
- 1: Vascular ingrowth
- 2: Osteoblasts lay new bone with simultaneous stochastic (aka random) resorption (Creeping Substitution)
- 3: Remodeling via cutting cones
- Cortical bone = Osteoclastic resorption (3 stages)
- In contrast to cancellous bone, NOT entire graft is incorporated and no remodeling occurs
- 1. Osteoclastic resorption via cutting cones into graft, causing 60% strength loss in first 6 months (All donor bone must be removed before new bone formation)
- 2. Initial incorporation at bone-graft junction via endochondral bone formation [Presence of Callus]
- 3. Ingrowth of osteons via intramembranous ossification / appositional bone growth
- Allografts undergo a similar process but more slowly
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- Quality of host bed - infection, vascularity, previous irradiation, immunocompromise
- Mechanical environment of graft - stability
- Inflammatory response - NSAIDs may delay response
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- Can harvest from anterior or posterior iliac crest - anterior yields less graft (15ml vs 25ml)
- Anterior approach - avoid LFCN
- Make skin incision parallel to iliac crest, 3cm proximal to ASIS to avoid LFCN
- Elevate EO muscles
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- RIA yields higher volume: 40ml vs 25ml
- (+) Lower overall pain scores
- (-) More blood loss, requires special instruments
- (-) Risk of iatrogenic fractures
RIA 2.pdf5314.1KB
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- Superior cluneal nerve blocks (L1-L3 dorsal rami pure sensory nerves)
ALLOGRAFTS
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- Two components of processing
- Aseptic processing - physical and sonic debridement, followed by ethanol and antibiotic treatment, then packaging in final container
- Terminal Sterilization
- Performed while tissue is in its final package
- Methods include ethylene oxide, gamma radiation, electron (E)-beam radiation, and hydrogen peroxide plasma
- Gamma radiation is most commonly used
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- Fresh vs Fresh frozen vs freeze dried
- Fresh - Stored at 4°C for 1 week; highest antigenicity with viable cells and growth factors
- Fresh frozen - Less immunogenic than fresh, preserves BMP; lasts 1 year at -20°C, 5 years at -70°C
- Freeze dried - Least immunogenic, reduced structural integrity, depleted BMPs; indefinite shelf life
- Cryopreservation - Cell viability 10-40% when stored at -160°C in glycerol medium
DBM
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- It is bone processed with acid extraction to remove minerals , leaving behind the collagenous and non-collagenous proteins and structure
- Available in gel and putty forms
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- Has osteoconductive properties. Its osteoinductive potential remains debated.
BMP
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- It is an osteoinductive growth factor that stimulates bone growth
- BMP-2 and BMP-7 are commonly used, primarily in tibial fractures
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- Early complications
- Seroma formation requiring extended drain placement; avoid use in cervical spine surgeries
- Can cause radiculitis - inflammation around nerve roots. (Note: in ALIF, irritation of the hypogastric nerve can cause retrograde ejaculation)
- Late complications
- Can cause ectopic bone formation leading to compression
- Can lead to osteolysis
- Risk of malignancy, avoid in patients with cancer risk
- Contraindicated in skeletally immature patients
BONE SUBSTITUTES
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- Calcium sulfate - absorbs within weeks!
- Calcium phosphate - available as ceramic cement
- Calcium Carbonate - Coralline/Hydroxyapatite.
- Tricalcium Phosphate [Chronos] - converts to hydroxyapatite after implantation
- Provides structural support and resorbs over 6-18 months
- Available in various sizes and shapes
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- Chronos - Tricalcium phosphate bone blocks ➔ partially converts to Calcium Hydroxyapatite, resorbs over 6-18 months (unlike calcium sulfate which absorbs within weeks and cannot provide structural support)
- Norian - calcium phosphate
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Keep hold of instruction; do not let go; guard her, for she is your life. Proverbs 4:13